Immune function is highly dependent upon costimulatory signals delivered during both the initial process of T cell activation and reactivation with entry into target tissue. There is a key role of costimulatory signals in autoimmune disease with activation of autoreactive T cells which has become evident with the emergence of autoimmmunity after blockade of coinhibitory signals in cancer. Moreover, gene variants in the costimulatory receptor CD226, and its ligand CD155 (shared with TIGIT) are strongly associated with risk of MS, suggesting a central role in MS autoimmunity. We discovered that TIGIT signaling in CD4+ cells induces IL-10, while CD226 is associated with IFN?/IL-17 secretion. Moreover, we identified Foxp3+ T cells expressing TIGIT as a distinct Treg subset with increased suppressor function. New preliminary data revealed that the CD226? TIGIT+CD4+ subpopulation from MS patients did not secrete IL-10 when compared to controls. Additionally, TIGIT was revealed to signal through AKT in MS Tregs with development of dysfunctional IFN? secreting Tregs. These new data have led us to deeply examine the function of TIGIT and CD226 on human T cells. We hypothesize that loss of TIGIT expression or function in MS drives autoimmune responses, while in tumors, increased TIGIT expression or signaling allows tumors to escape immune surveillance. We will determine the role of TIGIT/CD226 in regulating CD4+ T cell activation in human inflammatory disorders. First, we propose to test the hypothesis that the CD226/TIGIT axis regulates balance between IFN?/IL-17 vs. IL-10 production for T cells. Specifically, we propose that loss of signaling through the TIGIT pathway on MS T cells leads to loss of IL-10 while this pathway is hyperactive in glioma T cells. Serial RNAseq, ATACseq, and MINT-ChIP experiments will be performed after stimulation with TIGIT & CD226 mAbs in relationship to cytokine secretion, enabling us to elucidate key regulatory elements and associated epigenetic changes defective in TIGIT signaling in MS as compared to glioma. shRNA/CRISPR/Cas9 knockdowns will validate key regulatory nodes controlling proinflammatory vs. regulatory cytokines. Secondly, we will investigate the role of TIGIT/CD226 on the function of regulatory T cell populations in MS. In this aim, we hypothesize there are defects in the function of TIGIT+ Tregs in MS patients inducing dysfunctional Th1-like Tregs. We will measure frequency and function of TIGIT+ T cells in MS, determining whether TIGIT signaling prevents Th1 reprogramming of Tregs through repression of the AKT pathway. Using agonistic anti-TIGIT mAbs, we will interrogate molecular pathways of TIGIT in Tregs using RNA-seq, ATAC-seq, and MINT-ChIP serially after stimulation as readouts of biologic function allowing the elucidation of key regulatory elements that drive biologic processes which we will confirm by CRISPR/Cas9 and shRNA gene knockdown. In total, these studies will provide insight into the role of costimulatory pathways in the regulation of self- and tumor reactive T cells in autoimmune disease and cancer and potentially direct the development of new therapeutic approaches.